Staphylococcus aureus is a gram-positive facultative aerobe which is considered among the most virulent species of the genus. It is part of the bacterial flora indigenous to humans. A Staphylococcus aureus infection may result when injury occurs to the skin or protective mucous membranes, thus allowing invasion of the bacterium. Epidermal infection is the most common form of Staphylococcus infection in humans. However, Staphylococcus aureus is also a common organism isolated from patients with nosocomial pneumonia.
When introduced in food, Staphylococcus aureus may produce one or more staphylococcal enterotoxins. If ingested, heat stable Staphylococcal enterotoxins may produce symptoms of food poisoning and a range of other diseases.
Staphylococcus aureus possesses a resistant cell wall which comprises a cross-linked peptidoglycan layer which may protect the cell from invasion by the host. In addition, the cell wall is resistant to phagocytosis which is thought to be due, in part, to the production of Protein A on the cell surface. Staphylococcus aureus also produces hemolytic toxin which may damage blood cells and immune cells.
Treatment of Staphylococcus aureus infections generally comprises the use of penicillin, erythromycin, or methicillin. However, numerous strains have emerged which are resistant to penicillin and, recently, others have been found which are methicillin-resistant. These methicillin-resistant strains are usually resistant to most other anti-microbial drugs and therapy in response to a methicillin resistant strain usually requires administration of a more potent antibiotic, such as vancomycin.
Of interest to the field as well as to the present invention, are laboratory tests to identify Staphylococcus aureus infection and, in particular, methicillin-resistant strains which may not be detected by traditional assays and may be unresponsive to traditional therapy. Assays for Staphylococcus aureus typically are based upon (1) a coagulation assay, measuring coagulation of test serum to appropriately prepared rabbit plasma; (2) observation of the morphology of the suspected pathogen; (3) detection of the production of thermostable nucleases; and (4) the utilization by the pathogen of specific sugars as carbohydrate sources. Kloos, et al., Manual of Clinical Microbiology, 222-237 (Balows, et al., eds. 1991). Such traditional methods suffer from serious drawbacks. Traditional assays for S. aureus require several hours of incubation and may provide ambiguous results. Slide agglutination assays have been developed which may overcome some of those problems. Aldridge, et al., J. Clin. Microbiol., 19: 703-704 (1984); Baker et al., J. Clin. Microbiol., 21: 726-729 (1985); Berger-Ba/ chi, et al., Mol. Gen. Genetics, 219: 263-269 (1989). Such assays involve particles coated with fibrinogen or fibrinogen and an IgG immunoglobulin. Fibrinogen-binding protein (clumping factor) and protein A, both associated with the cell surface of S. aureus, bind to the coated particles, resulting in a positive assay. However, reports indicate that most such assays are not capable of detecting certain methicillin-resistant S. aureus. Berger-Ba/ chi, Supra; Laircsey, et al., J. Clin. Microbiol., 25: 181-182 (1987); Piper, et al., J. Clin. Microbiol., 26: 1907-1909 (1988); Wagner, et al., J. Clin. Microbiol., 30: 2583-2588 (1992).
Failure to detect methicillin-resistant strains may result in misdiagnosis and/or improper therapy. Also, to control the spread of S. aureus-related diseases and to determine the source of an outbreak of S. aureus infection, proper and precise identification of the bacterium, including the strain, is essential.
Accordingly, there is a need in the art for rapid, reliable means for detecting all methicillin-resistant strains of Staphylococcus aureus, as provided in the present invention